Leucocytes, or white blood cells, refer to cells in the haematopoetic system that are distinguished by their overall colour and appearance. Specifically, they appear 'white' in comparison with red cells (erythrocytes) and are much larger than platelets (which are technically fragments of a much larger precursor cell, the megakaryocyte). As a very broad term, it hides the significant complexity and variety of the different subtypes.
Historically, based on morphological appearance (as early research was based on microscopy studies).
Knowledge of haematopoetic lineage, molecular surface markers and their roles within immunity and inflammation permit an alternative, and perhaps more meaningful classification.
These cells all contain granules, which stain with different dyes.
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